1. Field of the Invention
This invention relates generally to acoustic absorbers. More particularly, this invention relates to lightweight acoustic absorbers that absorb energy during launch of a vehicle by a rocket, and which generally prevent contamination of a fairing containing the vehicle.
2. Description of the Related Art
Since the beginning of the United States Space Program in the '60s, payloads such as satellites have been lifted by expendable and non-expendable launch vehicles into orbit and beyond. Typically, a payload fairing is provided to house the vehicle or satellite and to prevent damage to the vehicles therein before and after launch. Fairings have been used extensively in the payload lifting art, and will be continued to be used as this art progresses to more sophisticated land based launches and the new expected sea launch of space vehicles and payloads.
Fairings and payloads are typically mounted to the top of the rocket and are thus subjected to severe forces. Aerodynamic, heat, and acoustic forces all combine to produce a stressful and mechanically challenging environment for the satellite within the fairing. In particular, acoustic emissions which occur in connection with the launch of the vehicle, and which are produced by the thrust of the rocket, can exceed 140 dB. Such intense acoustic loads can damage the satellite within the fairing and contribute to the degradation of performance of satellite subsystems such as solar cells, control systems, and communication systems.
In order to alleviate some of these problems, it has been known in the art to use acoustic protection on the inside of fairings. See, for example, U.S. Pat. No. 5,670,758, Borchers et al., the teachings of which are expressly incorporated herein by reference. In the Borchers, et al. patent, acoustic panels are tuned to absorb energy in a defined frequency range and are disposed on foam plastic mats and fastened on the inside of the payload fairing. A complex homed structure having tapered ends and curved conical members form the acoustic panels of Borchers et al. and are intended to reduce the acoustic load experienced by the fairing before and during launch.
The panels of the Borchers et al. patent comprise a series of foamed plastic mats of a reticulated polyimide for example, the polyimide sold under the trade name Solimide AC406 available from the Illbruck firm in Switzerland. Other polyimide foams are also usable in acoustic absorbers. One of the major problems with these types of foams however is that they tend to emit particles into the atmosphere of the fairing which contaminate the atmosphere and can settle on portions of the payload, thereby damaging or impeding the performance of the payload during launch and later on in orbit. In order to address this problem, the absorbing polyimide foams of the Borchers et al. patent are covered by protective foils, which are provided in an attempt to prevent leakage of the contaminants from the foam into the fairing atmosphere.
In order to accommodate changes of atmospheric pressure in the fairing during launch that causes the absorbing panels to expand, fine perforations are placed in the protective foils of Borchers et al. to allow for the differential pressure. However, the thin protective foil is not placed around the entire absorbing foam in the Borchers et al. patent and the sides of the panel which are not covered by the foil must be adhered to the fairing surfaces directly which allows for outgassing from the foam to occur and for the panel to become easily detached from the fairing wall which is threatening to the payload.
The sound absorbing panels of the Borchers et al. patent require complicated structures to be formed out of the polyimide material. Furthermore, since the thin protective foil which covers one side of the panels of Borchers et al. does not encompass the entire foam structure, there still remains the dangerous possibility of a leak of contaminants into the fairing's atmosphere and vice-versa. The requirement that the entire side of the supporting surface of the foam plastic mats be adhered to the inside of the fairing of Borchers et al. also allows for the strong possibility that the foamed material will be exposed to the atmosphere of the fairing during launch should the foam shake loose from the fairing, or not be adhered correctly and efficiently during the adhesion process. Additionally, the use of fine perforations in the protective foil will also allow gases to escape into the fairing atmosphere and does not provide for adequate differential pressure compensation and further increases the possibility of contaminating the fairing's atmosphere with the foamed material.
Prior acoustic absorbing panels have been secured to the inside walls of fairings with mechanical fasteners such as VELCRO, grommets, screws and brads. This way of securing acoustic absorbing panels to the fairing is inefficient and dangerous since during liftoff the panels can become disengaged from the fairing surface due to the severe forces that are generated. Additionally, the use of mechanical fasteners greatly increases installation time and is therefore also economically inefficient. Further, the mechanical fasteners undesirably increase the weight of the acoustic absorbing panels. Thus, prior mechanical fastening of acoustic panels to fairings is not safe, economical or effective.
There thus remains a long felt need in the art for an effective acoustic absorbing panel that can efficiently and adequately ensure that the payload is not subjected to acoustic stresses before and during launch, and is not subjected to dangerous contaminants and funguses which are the natural result of the use of a sound absorbing foam. Such needs have not heretofore been fulfilled in the art.